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CNC Basics · Chapter 7

Deep holes

If you stir deep into a cup of ice with a straw, it clogs at some point. You pull it out, shake it and stir on. A drill in a deep hole has the same problem: the chips (the small pieces of metal that are cut away) have no room. That is why deep holes are drilled in steps.

Why chips are a problem

The drill cuts material. The chips travel up along the flutes of the drill. In a shallow hole they fall out quickly. In a deep hole they jam, the drill gets hot and may break. As a rule of thumb a hole is deep from about three times its diameter.

Retract completely: G83

The cycle G83 drills one step, retracts completely so the chips fall out, goes back in and drills the next step. It adds the word Q: the peck depth, the depth per step in mm. Q is always positive.

The machine requires at least Q0.5. The right peck depth depends on the material. Soft aluminium: about half to one diameter. Tough material: less.

G90 G21 G17T2 M6S1000 M3G83 X30 Y25 Z-24 R2 Q4 F50   (24 mm deep in steps of 4 mm, full retract)G80M5M30

Breaking chips without retracting: G73

Retracting completely takes time. Sometimes it is enough to break the chips: after each step the drill backs off only a little, with G73 that is 0.5 mm. The chip breaks off, the drill stays in the hole.

G73 is faster, G83 clears more thoroughly. Very deep holes are drilled with G83, tough material at medium depth with G73.

G90 G21 G17T2 M6S1000 M3G73 X30 Y25 Z-24 R2 Q4 F50   (24 mm deep in steps of 4 mm, small lift)G80M5M30

Several deep holes

G83 and G73 are modal like G81. The values for Z, R, Q and F appear only in the first block, after that the position is enough.

G90 G21 G17T2 M6S1000 M3G83 X20 Y25 Z-24 R2 Q4 F50   (the cycle starts)X45                          (same depth, same step)X70G80M5M30

Common mistakes

  • Forgetting Q

    G83 and G73 without Q raise an alarm. Q belongs in the block with the cycle.

  • Q too small

    A peck depth below 0.5 mm gives hundreds of tiny steps and only costs time. The machine refuses it.

  • Q too large

    If Q is almost as large as the whole hole, the chip does not break and you have wasted the benefit of the cycle. For aluminium, 3 to 6 mm are good values for small drills.

  • Not ending the cycle before a tool change

    End the cycle with G80 and switch the spindle off with M5 before you change the tool with M6.